The present invention relates to a mask for use in photolithography and its manufacturing technique, and particularly to a technique effectively applicable to a mask for use of manufacturing semiconductor integrated circuit device.
In recent years, very fine elements constituting a circuit, very fine wirings and very narrow spaces between the elements and wirings have been developed in semiconductor integrated circuit devices.
However, along with such development of the elements and wirings and of the spaces between elements and wirings, there arises a problem in that the accuracy of mask pattern transfer is lowered when an integrated circuit pattern is transferred onto a wafer by coherent light.
This problem will subsequently be described with reference to FIGS. 24 (a)-(d).
When a given integrated circuit pattern formed on a mask 50 shown in FIG. 24 (a) is transferred onto a wafer by a method of projection exposure or the like, the phases of lights each transmitted through each of a pair of transmission regions P.sub.1 P.sub.2 having light shield region N therebetween are identical to each other as shown in FIG. 24 (b). Consequently, these interferential lights increase their intensities in light shield region N located between the above-mentioned pair of transmission regions P.sub.1, P.sub.2 as shown in FIG. 24 (c). As a result, as shown in FIG. 24 (d) the contrast of a projected image on a wafer is not only lowered, but also the depth of focus becomes shallow, causing the transfer accuracy of the mask pattern to be considerably lowered.
As a means to counteract these problems, a technique of phase shifting lithography has been developed, whereby the phase of light transmitted through the mask is controlled so as to improve the resolution and contrast of the projected image. The phase shifting lithography technique is disclosed, for example, in Japanese Laid-Open Patent No. 73744/1983 and Japanese Laid-Open Patent No. 67514/1987.
In the above-mentioned Japanese Laid-Open Patent No. 173744/1983 there is described the structure of a mask having a light shield region and a pair of transmission regions, wherein a transparent material is arranged at least in either one of the transmission regions sandwiching the light shield region therebetween, allowing a phase difference to be generated between the lights each transmitted through each of transmission regions at the time of exposure and thus these lights being interfered with each other to weaken themselves in the region on a wafer which should primarily be a light shield region.
The function of the light transmitted through such a mask as above will subsequently be described with reference to FIGS. 25 (a)-(d).
When a given integrated circuit pattern formed on a mask 51 shown in FIG. 25 (a) is transferred onto a wafer by the method of projection exposure or the like, a phase difference of 180.degree. is generated between the phase of light transmitted through a transmission region P.sub.2 having transparent material 52 of a pair of transmission regions P.sub.1, P.sub.2 which have light shield region N sandwiched therebetween and the phase of light transmitted through the normal transmission region P.sub.1 as shown in FIGS. 25 (b) and (c). Therefore, the lights transmitted through the pair of transmission regions P.sub.1, P.sub.2 interfere with each other to offset them in light shield region N located between these transmission regions P.sub.1, P.sub.2. Consequently as shown in FIG. 25 (d) the contrast of a projected image on a wafer is improved. Thus, the resolution and depth of focus is improved, resulting in a higher accuracy of pattern transfer of the mask 51.
Also, in the above-mentioned Japanese Laid-Open Patent No. 67514/1987, there is described the structure of a mask having a light shield region formed by light shielding film and a transmission region formed by removing the light shielding film, wherein a fine aperture pattern is formed by removing a part of shielding film and at the same time, a phase shifting layer is provided on either one of the transmission region or the aperture pattern, and thus a phase difference is generated between the lights transmitted through the transmission region and the aperture pattern, preventing the distribution of amplitude of light transmitted through the transmission region from being spread in the horizontal direction.